Implantable devices require compact power sources and energy efficient electronics for prolonged device operation. Battery-powered devices must be explanted each time the power supply fails or reaches end of life ("EOL"). Ideally, the power management system of a battery-powered implantable device optimizes battery utilization by controlling battery consumption and by providing an elective replacement indicator ("ERI") which provides sufficient notice to the patient's doctor that end of life is near without prematurely declaring ERI.
Several methods have been suggested for an ERI, however, battery characteristics vary and an accurate ERI is needed in the art. Generally speaking, internal battery resistance increases as the battery is used, however, transient internal resistances have been observed which vary in a complex function of battery life and history of current draw. The internal battery resistance will be called the "steady state internal resistance" throughout this document to distinguish it from the transient internal resistance. As stated before, the steady state internal resistance does vary with current consumption, and is therefore a function of current drawn from the cell, but it changes due to mechanisms which differ from the transient internal resistance mechanisms.
Furthermore, none of the previous power management systems have addressed a specific problem found in lithium-silver-vanadium-pentoxide batteries which arises when current is extracted from the battery in certain portions of the battery life curve. A lithium-silver-vanadium-pentoxide battery exhibits an abrupt increase in internal resistance in certain periods of battery life due to formation of a "passivation layer" on the lithium surface following periods of relatively low current draw from the battery. The passivation layer creates a transient resistance which diminishes when current is drawn from the battery.
This effect is called "voltage delay" since the output voltage of the battery drops significantly upon current demand due to a large transient internal battery resistance. As current is extracted from the battery, the transient internal resistance is diminished and the output voltage of the battery returns to the ordinary output voltage for that portion of the battery lifetime.
The voltage delay effect may lower the battery output voltage below the reset voltage of the device electronics during high initial current draw. Even if the output voltage of the power source does not initially drop below the reset voltage the pulse delivery circuit may never draw enough energy to completely charge the high voltage output circuit due to a large steady state internal resistance of the battery.
Another problem with the previous power systems in implantable devices is that as the battery (or batteries, for multiple battery devices) approaches its EOL, the battery has increasing difficulty in providing adequate charge to the output capacitor to deliver a therapy pulse. Therefore, explantation and replacement of the device is performed earlier than necessary. The battery erroneously appears to have reached EOL because as current is drawn the voltage delay provides sufficient transient internal resistance to reduce the terminal voltage of the battery so as to signal elective replacement of the battery.
Therefore, there is a need in the art for an implantable power management system which extracts current from the power supply without requiring premature device replacement and without risking a reset of device hardware. The power system should also manage steady state internal battery resistance to ensure that the power supply can deliver a complete therapy pulse or, alternatively, signal the patient that the power supply is unsafe for charging the output circuits. There is yet further a need in the art for a power system which manages the transient internal battery resistance synonymous with the voltage delay effect so as to maximize battery life and reduce the number of device replacements.